fix(server): streamed section titles wait for the text under them

apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.test.ts

@@ -5174,4 +5174,59 @@ describe("splitBufferedAssistantText", () => {
       rest: "```\n- one\n- two\n",
     });
   });
+
+  it("holds a heading until the block under it is done", () => {
+    expect(splitBufferedAssistantText("intro\n\n## Setup\n\nInstall it")).toEqual({
+      ready: "intro\n\n",
+      rest: "## Setup\n\nInstall it",
+    });
+    expect(
+      splitBufferedAssistantText("intro\n\n# Plan\n\n## Setup\n\nInstall it.\n\nNext"),
+    ).toEqual({
+      ready: "intro\n\n# Plan\n\n## Setup\n\nInstall it.\n\n",
+      rest: "Next",
+    });
+  });
+
+  it("delivers the paragraph above a heading with no blank line between them", () => {
+    expect(splitBufferedAssistantText("para\n## Setup\n\nInstall")).toEqual({
+      ready: "para\n",
+      rest: "## Setup\n\nInstall",
+    });
+    // A bold line there continues the paragraph, so both stay buffered.
+    expect(splitBufferedAssistantText("para\n**Setup**\n\nInstall")).toEqual({
+      ready: "",
+      rest: "para\n**Setup**\n\nInstall",
+    });
+  });
+
+  it("holds a line of only bold text like a heading", () => {
+    expect(splitBufferedAssistantText("**Risk by area:**\n\n| a |\n|---|\n")).toEqual({
+      ready: "",
+      rest: "**Risk by area:**\n\n| a |\n|---|\n",
+    });
+    expect(splitBufferedAssistantText("**Use *npm* now**\n\nInstall it")).toEqual({
+      ready: "",
+      rest: "**Use *npm* now**\n\nInstall it",
+    });
+    expect(splitBufferedAssistantText("**Note:** read this.\n\nNext")).toEqual({
+      ready: "**Note:** read this.\n\n",
+      rest: "Next",
+    });
+  });
+
+  it("delivers a held heading with its first list item or its whole code block", () => {
+    expect(splitBufferedAssistantText("## Steps\n\n- one\n- tw")).toEqual({
+      ready: "## Steps\n\n- one\n",
+      rest: "- tw",
+    });
+    expect(splitBufferedAssistantText("## Code\n\n```ts\na\n\nb\n")).toEqual({
+      ready: "",
+      rest: "## Code\n\n```ts\na\n\nb\n",
+    });
+    expect(splitBufferedAssistantText("## Code\n\n```ts\na\n```\nafter")).toEqual({
+      ready: "## Code\n\n```ts\na\n```\n",
+      rest: "after",
+    });
+  });
 });

apps/server/src/orchestration/Layers/ProviderRuntimeIngestion.ts

@@ -211,6 +211,12 @@ const BLANK_LINE_PATTERN = /^[ \t]*$/;
 // nested items count. The trailing space is required, so a partial `-` or
 // `1.` never matches before the model finishes the marker.
 const LIST_ITEM_START_PATTERN = /^[ \t]*(?:[-*+]|\d{1,9}[.)])[ \t]/;
+// A section title: an ATX heading, or a line of only bold text, which models
+// often use as a heading.
+const SECTION_TITLE_PATTERN = /^ {0,3}(?:#{1,6}(?:[ \t]|$)|\*\*(?:[^*]|\*(?!\*))+\*\*:?$)/;
+// An unindented ATX heading ends the paragraph or list above it, even with no
+// blank line between them. A bold line would continue the paragraph instead.
+const TOP_LEVEL_HEADING_PATTERN = /^#{1,6}(?:[ \t]|$)/;
 
 /**
  * Splits buffered assistant text at the last blank line, closing code fence,
@@ -221,17 +227,26 @@ const LIST_ITEM_START_PATTERN = /^[ \t]*(?:[-*+]|\d{1,9}[.)])[ \t]/;
  * never leaks; a list item start is the one lookahead that may sit on the
  * partial line, since tight lists have no blank lines between items and would
  * otherwise land all at once.
+ *
+ * A section title holds the boundary until a content line follows it, so a
+ * title never lands alone and waits above a block that is still streaming.
  */
 export function splitBufferedAssistantText(text: string): { ready: string; rest: string } {
   let openFence: { marker: string; indent: number } | null = null;
   let boundary = -1;
   let lineStart = 0;
+  let titleAwaitingContent = false;
   for (;;) {
     const newline = text.indexOf("\n", lineStart);
     const line = text
       .slice(lineStart, newline === -1 ? text.length : newline)
       .replace(/[ \t\r]+$/, "");
-    if (openFence === null && lineStart > 0 && LIST_ITEM_START_PATTERN.test(line)) {
+    if (
+      openFence === null &&
+      lineStart > 0 &&
+      !titleAwaitingContent &&
+      LIST_ITEM_START_PATTERN.test(line)
+    ) {
       boundary = lineStart;
     }
     if (newline === -1) {
@@ -243,6 +258,7 @@ export function splitBufferedAssistantText(text: string): { ready: string; rest:
       const marker = fenceMatch[2]!;
       if (openFence === null) {
         openFence = { marker, indent };
+        titleAwaitingContent = false;
       } else if (
         marker[0] === openFence.marker[0] &&
         marker.length >= openFence.marker.length &&
@@ -254,7 +270,14 @@ export function splitBufferedAssistantText(text: string): { ready: string; rest:
         boundary = newline + 1;
       }
     } else if (openFence === null && BLANK_LINE_PATTERN.test(line) && lineStart > 0) {
-      boundary = newline + 1;
+      if (!titleAwaitingContent) {
+        boundary = newline + 1;
+      }
+    } else if (openFence === null) {
+      if (lineStart > 0 && !titleAwaitingContent && TOP_LEVEL_HEADING_PATTERN.test(line)) {
+        boundary = lineStart;
+      }
+      titleAwaitingContent = SECTION_TITLE_PATTERN.test(line);
     }
     lineStart = newline + 1;
   }